We have isolated immediate early (IE) viral messenger (m) RNA from vero cells infected with Herpes Simplex Virus (HSV) in the presence of cycloheximide (cx). An mRNA fractionated by density gradient sedimentation enriched by oligo dT-cellulose chromatography was translated. IE mRNA cell free and specific polypeptide products were identified. Specifically, we obtain proteins in a cell free system of 175,000, 110,000 and 68,000 daltons which are characteristic of IE HSV specific products seen with cells labeled in vivo. We have similarly obtained from defective HSV infected vero cells a species of IE mRNA and have compared products in a cell free translation to the nondefective HSV mRNA specific polypeptides. Both mRNAs specify the polypeptide products of 175,000, 110,000 and 68,000 daltons, however the levels synthesized of 175,000 dalton product using a comparable quantity of mRNA is 2-3-fold greater with defective mRNA. Vmw 135 seen in the non-defective IE mRNA cell free translation system was markedly diminished in the defective IE mRNA translation.